Sound-evoked activity plays a critical role in the development of the auditory system. Likewise, auditory deprivation resulting from deafness causes abnormal development at many levels. Previous work has shown that congenitally deaf animals, including the deaf white cat (DWC), have morphologic abnormalities at the endbulb of Held synapse, an important central synapse of the auditory nerve. The proposed research will explore the functional implications of these morphologic abnormalities, the sensitivity of the synapse to deafening at various ages, and the extent to which the changes can be prevented or reversed by artificial stimulation of the auditory nerve with a cochlear implant. The long-term goal is to define a critical period during which the auditory system is sensitive to deafness. It has long been known that postlingually deafened adults, and deaf infants implanted early, receive more benefit from cochlear implantation than congenitally deaf individuals implanted as adults. The abnormalities seen at the endbulb synapses in deaf mammals may cause abnormal activity in higher auditory centers, and could explain the difficulties deaf adults have in interpreting auditory nerve stimulation. Thus, the proposed studies are not only relevant to general neurobiological issues of auditory plasticity and development, but also to mechanisms of rehabilitation in deaf humans.